1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus incorporating a fixing device that fixes a toner image in place on a recording medium with heat and pressure.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such fixing device includes a roller-based fuser assembly that employs a fuser roller equipped with an internal heater to heat its circumference to a given process temperature. The fuser roller is paired with a pressure roller pressed against the outer circumference of the fuser roller to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser roller and pressure from the pressure roller.
Another type of fixing device includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt entrained around multiple rollers, one of which is equipped with an internal heater to heat the length of the fuser belt through contact with the heated roller. The fuser belt is paired with a pressure roller pressed against the outer surface of the fuser belt to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser belt and pressure from the pressure roller.
One problem common to those types of fixing device is variations in a linear conveyance speed with which the recording medium is conveyed through the fixing nip along the circumference of the rotary fixing member. The problem arises where the fixing member is formed of thermally expansive material, such as a rubber-based fuser roller or the like, which contracts and expands as the fixing device operates under varying operating temperatures, resulting in variations in diameter, and hence circumference, of the rotating fixing member.
For example, in a belt-based fixing device employing a motor-driven fuser roller around which a fuser belt is entrained, the fuser roller has its diameter gradually increased as the rubber-based outer layer thermally expands due to heat from the fuser belt subjected to heating during operation. Where the fuser roller is driven with a constant rotational speed or frequency, variations in the roller diameter translate into variations in the conveyance speed with which a recording medium is conveyed along the circumference of the fuser roller. That is, an increase in the roller diameter yields a faster conveyance speed, whereas a decrease in the roller diameter yields a slower conveyance speed.
Although such problem is experienced by a roller-based fixing device as well, the difficulty is more pronounced in the belt-based design than in the roller-based design, since the former typically employs a thick rubber-covered fuser roller with no dedicated heater provided therein, which is susceptible to variations in temperature, and therefore is prone to thermally-induced variations in circumferential conveyance speed, particularly in applications for high-speed color printers.
In a media conveyance path, the fixing process is followed by a post-fixing process, such as an output unit for outputting a recording medium to a subsequent process, or a secondary fixing unit for processing a toner image subsequent to processing through the fixing nip. Such post-fixing mechanism typically has a regulated, substantially constant speed compared to that of a fixing device. This is particularly true of a secondary fixing device formed of a compact, thin rubber-covered roller assembly designed to impart gloss on a printed image after fixing, which is relatively immune to thermally-induced dimensional variations, and concomitant variations in circumferential conveyance speed.
Not surprisingly, where a post-fixing process conveys a recording medium with a constant conveyance speed, variations in conveyance speed in the fixing device result in a difference or inconsistency between the fixing and post-fixing media conveyance speeds. If not corrected, such speed differential (or variations therein) can affect imaging quality as well as media conveyance performance downstream from the fixing nip along the media conveyance path.
For example, where the fixing device processes a recording medium with a conveyance speed significantly slower than that of the post-fixing process, the recording medium, advanced faster at its downstream, leading edge than at its upstream, trailing edge, rubs or strikes against a paper stripper or a similar guide mechanism, thereby causing image defects during conveyance downstream from the fixing nip.
On the other hand, where the fixing device processes a recording medium with a conveyance speed significantly faster than that of the post-fixing process, the recording medium, advanced faster at its upstream, trailing edge than at its downstream, leading edge, slacks into a bow which then creates accordion-like folds to jam the media conveyance path downstream from the fixing nip.
To counteract the problem, various methods have been proposed to maintain the speed differential within a specified acceptable range, so as to convey a recording medium in an appropriately slack, unstrained state between the fixing and post-fixing processes along the media conveyance path.
For example, one such method proposes an image forming apparatus incorporating a belt-based fixing assembly, in which an endless fuser belt is entrained around a fuser roller and a heat roller internally heated with a lamp, while paired with a motor-driven pressure roller pressed against the fuser roller via the fuser belt to form a fixing nip therebetween.
According to this method, a speed controller is provided to control a rotational speed or frequency of a rotary motor driving the pressure roller. Such rotational speed control is performed according to readings of a thermistor detecting temperature of the fuser belt, so as to rotate the pressure roller at a constant circumferential speed irrespective of variations in operating temperature of the heat roller.
Another method proposes an image forming apparatus incorporating a fixing device disposed downstream from a transfer process that transfers a toner image onto a recording medium from another imaging surface.
According to this method, a slack detector is disposed between the transfer and fixing processes to detect slack of a recording medium being conveyed with its leading edge entering the fixing nip and its trailing edge still remaining in the transfer process. Readings of such slack detector are transmitted to a speed controller, which accordingly controls a rotational speed or frequency of a rotary motor driving a pressure roller, so as to control a media conveyance speed through the fixing nip depending on the amount of slack experienced by the incoming recording medium.
Further, the speed controller is equipped with a pair of first and second thermistors disposed at a circumference of the pressure roller, the former facing the fixing nip, the latter opposite the fixing nip. The speed controller adjusts the media conveyance speed according to readings of the first thermistor indicative of thermal expansion of an adjoining fixing member. Also, the speed controller determines an expected amount of expansion of the pressure roller according to readings of the second thermistor detecting temperature of the pressure roller.